I have used them many years ago and they work fine. I believe the west micro lite 410 is a plastic of some type. micro. It is a lot more expensive and I did not find an advantage, seems to sand about the same. A litter harder to mix into the resin and seems to cake up in moist air. but I would say it can't hurt to try some as they may have made some improvments since I tried the stuff.

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This is my opinion of these facts and only my opinion, your opinion may vary

Lynn Erickson A&P for lets say almost 30 years
Much better with a Dremel than a computer.
What if they gave me choice between a fast computer or a fast plane?

for what it's worth, I just picked up a batch, and I'm going to try it out. Dry weight's a little lower then K20 glass balloons, but it all depends on how well it mixes I guess. Supposed to have a higher shear strength, which is where my interest is from, given I'm hardshelling most of my foam, I want to try and recover the (debated) margin lost in this, and this might help do so. Three times the price of glass balloons though.

Anyone else have any experience doing something similar in the year-ish since this thread was last posted too?

I tried it long ago with MGS. For some reason, it slowed the cure of the micro to a crawl , so I tossed it in favor of the known properties of glass micro. I hear it works with West though.

Well, I'm running an experiment, and I noticed something that might explain some of the above...

I mixed up a batch of very, very (probably too) dry micro (but what do I know, I've not mixed finishing micro before), about 6:1 by volume, this isn't for use on the plane, just to see how the two wet and such. It was very flakey, but smoothed when pressed into a plastic cup, one batch with glass balloons and the other with phenolic. I used 25g 285 resin + 10g fast hardener for this.

What was interesting is that because of the all-fast and leaving the micro in the pot to cure, it exothermed fairly quickly... and the glass micro both exothermed noticably quicker than the phenolic, and was much more difficult to control when it did. Basically, I had to keep the glass micro cup running under cold water most of the time, while the phenolic only required occasional cooling for a minute at a time.

Perhaps the reason that the phenolic seems to be taking so much longer to cure is that glass bubbles are a better insulator (perhaps, it seems, much better), so the phenolic is allowing the internally generated heat from curing to dissipate, whereas the glass micro isn't and so is being accelerated?

Anyhow, I'll make another post when the experiment is finished, I'm just trying to figure out if there's significant final density differences.

I finished up the little experiment, I'm just going to cut and paste the notes I made for my build log rather than rewrite it, so a little is repetative of the above...
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I've been curious about how phenolic microballoons would compare to the standard glass (K20) balloons, so when I was last at my composites supplier, picking up MGS, I also picked up some phenolic balloons. Before I used them on my plane, I wanted to compare them to the plans specified stuff.

Basically, my concerns are:

Weight, can this stuff be effectively mixed to the same density as the regular micro

Hardness/Sandability/Durability, is it similar to glass micro

Water absorbtion, is it going to absorb water over time

Usability, is it as good or better than glass

On opening up the box, I can immediately say, it doesn't tend to become airborne quite as easily as glass micro, but it's still pretty bad for that. It's a little more clumpy, so gentle disturbances don't make it airborne, but scooping it still does.

I mixed up two batched of very dry micro, mixed about 1:6 by volume (actually about 1:6¼). I used 35 grams of epoxy, and then filled to about 215 mL a plastic cup for each of phenolic and glass – both wieghed 49 grams after mixing, so fairly close to 5:2 by mass. The glass seemed to mix a little more easily at first, but seemed to be drier at the end, and was slightly more in volume, although I later noticed that the phenolic “rose” sort of like bread as it cured (likely expansion due to the exotherm of the fast epoxy) and the glass may have done the same without me noticing it (it exothermed much faster than the phenolic did). On the whole, from a handling perspective, I think it's about a tie, with the phenolic being just as usable, perhaps slightly more so, based on this admittedly very limited trial.

As I mentioned, the phenolic mixed a little flatter, though, strangely, it “rose” as it cured, and possibly the glass did the same before I noticed, so given it didn't sink later after cure, it's probably a result of thermal expansion during exotherm, so it probably wouldn't do this in normal circumstances, unless the material was strongly heat cured. The first two pictures show this, with the line where the “dry” bubbles were added to before mixing.
The first picture was before the phenolic started to cook off. After cure (second picture), both are at about the same level. I finished both off at 70C in the oven for 45 minutes, and the phenolic finished exotherming and slightly deformed the cup. I popped the pieces free of the plastic (picture three), the phenolic peeled off slightly coarser in texture (possibly due to having taken the PE cup to Tg in the oven). The phenolic piece weighed at 48g and the glass 49g, so within experimental error, essentially the same. I sanded both to the same 40 gram weight (see picture four).
The phenolic was slightly tougher to sand, in that the belt sander took a slightly longer time to chew the surface flat, but not by much. Interestingly, the glass micro has that familiar, very slightly chalky feel, while the phenolic micro feels decidedly like a piece of plastic.

I used an old geology trick to try and determine which was harder, trying to scratch one with the other. The glass micro marks the phenolic more easily than the reverse, and the micro corner I used was more worn down by the scratching on the glass micro than the phenolic corner, so I'd give the edge in hardness to the phenolic, and probably toughness too, although that's more a guess. Both edges can be marred with a fingernail after the limited postcuring I gave it.

I dropped both into full glasses of water to float them, and try to get an idea of which floated higher (i.e. is less dense), I think that there's a slight advantage to the phenolic micro here, but it's small, maybe not even outside of experimental error (picture seven).

To check the water absorbsion (in a very cursory way), I dumped both 40 gram pieces into boiling water and boiled them covered for 15 minutes. I then dried them with a paper towel, and found they were 42 grams, both. Some of this is probably surface absorbed water in the open pores, rather than really absorbed, a much longer soak is probably needed for real data. Drying for a further 15 minutes at 110C brought them both back down to 40 grams again.

Given the boiling and drying probably aged the cure a bit, I checked the corner with a fingernail again, and the glass micro was more chalky than the phenolic and while both could be chipped off, the glass micro chipped more easily. Using a screwdriver bit to indent the surface, the glass micro is a little harder, but damages more when enough force is used to indent it.

So, I think, as far as I can tell, micro made with phenolic is, to my initial quick look anyhow, as good as glass micro, and I'm going to start using it exclusively over glass micro on my plane unless something turns up to indicate otherwise. Particularly, I want to see how well it works with slow micro, so, given I've about 193 nail holes in my fuselage bottom to fill, maybe I'll try it out there.
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for what it's worth, I just picked up a batch, and I'm going to try it out. Dry weight's a little lower then K20 glass balloons, but it all depends on how well it mixes I guess. Supposed to have a higher shear strength, which is where my interest is from, given I'm hardshelling most of my foam, I want to try and recover the (debated) margin lost in this, and this might help do so. Three times the price of glass balloons though.

Anyone else have any experience doing something similar in the year-ish since this thread was last posted too?

It would be worthwhile to test it with a variety of epoxies. I tried it years ago with the old Epolite (Safetypoxy) and it took 2 weeks to harden. But it was fine with WEST.

Shear strength is a non-issue. No matter what epoxy/fill mix you use, it's going to be MUCH stronger than the foam.

Shear strength is a non-issue. No matter what epoxy/fill mix you use, it's going to be MUCH stronger than the foam.

For a hand layup done as per plans, glass over micro slurry, I'd agree.

However, I'm vac bagging, and found that if I bag over micro slurry, it migrates up into the laminate. As a result I'm hard shelling, and I don't get completely consistant foam-micro interface failure when I test this (admittedly I'm testing in peel, not shear). Additionally, mixing the micro thicker (I'm probably using something closer to wet micro at about 2.5:1 than slurry at 1:1 v/v) based on some advice I saw here to thicken the micro to prevent bleed, seems to give me a better fill, though of course this reduces strength. I'm going to be mixing my micro for glass layups thinner and the idea is the additional strength of the phenolic balloons will offset the loss in hardshelling. This is of course almost completely a qualitative assessment, and so it's could be anywhere from unnecessary to insufficient, but my feel is it's much closer to the former than the latter.

I had really hoped there would be a density difference between the two, but that seems pretty small.

Anyone know about how many pounds or gallons of micro get left on (as opposed to applied but sanded off) the average Cozy?

Burt Rutan indicated in the arly RAF tape on composite methods that phenolic microballoons would preferentially draw one of the agents out of the hardener in the then preferred epoxy, and inhibit cure. I have to suspect that this characteristic may be present with some of the current epoxies as well.

I do find it significant that Gougeon sells Phenolic microballoons under their West brand, and that West brand epoxies work OK with it...